This invention relates to enzymatic processes for converting glucose (dextrose) to fructose (levulose).
Most food grade glucose is provided as an enzymatic hydrolysate of corn starch, i.e., the corn syrup of commerce. Glucose is generally rated as being 60 to 80% as sweet as sucrose and therefore sells at a correspondingly lower price. It has long been known to isomerize glucose to fructose which is even sweeter than sucrose by employing an enzyme having glucose isomerase activity, preferably one which has been immobilized by adsorption onto insoluble supports, such as crosslinking the enzyme with the support matrix and entrapment in a polymer matrix support such as diethylaminoethylcellulose or porous glass. The isomerization of glucose provides an equilibrium mixture typically containing 42-50% fructose and is referred to as high fructose corn syrup (HFCS).
It is known that glucose isomerase can be isolated from a substantial number of microorganisms including species of Streptomyces, Bacillus, Arthrobacter, Nocardia, Lactobacillus, Ampullariella, and various other genera of microorganisms, and the enzyme has been employed in the commercial production of fructose from glucose by known isomerization techniques to provide mixtures of glucose and fructose. In the commercial process most commonly in present use, cornstarch is liquefied, enzymatically or chemically, and then treated with glucoamylase to produce glucose which is thereafter isomerized using glucose isomerase to mixtures containing both fructose and glucose. Higher concentrations of fructose are particularly desirable and may be obtained by the use of more active enzymes and/or the use of high isomerization temperatures.
Detailed descriptions of the enzymatic conversion of glucose to fructose employing glucose isomerase can be found in Hamilton, et al. "Glucose Isomerase: A Case Study of Enzyme-Catalyzed Process Technology", Immobilized Enzymes in Food and Microbial Processes, Olson et al., Plenum Press, New York, (1974), pp. 94-106, 112, 115-137; Chen, et al., "Glucose Isomerase (a Review)", Process Biochem., (1980), pp. 30-35; Chen, et al., "Glucose Isomerase (a Review)", Process Biochem., (1980), pp. 36-41; Nordahl, et al., "Fructose Manufacture from Glucose by Immobilized Glucose Isomerase", Chem. Abstracts, vol. 82, (1975), Abs. No. 110316h; and, Takasaki, "Fructose Production by Glucose Isomerase", Chem. Abstracts, vol. 81, (1974), Abs. No. 76474a. In addition, there are numerous patents relating to glucose isomerization of which U.S. Pat. Nos. 3,616,221; 3,623,953 (Reissue 28,885); 3,964,313; 3,708,397; 3,715,276; 3,788,945; 3,909,354; 3,960,663; and, 4,308,349 are representative.
Because of the economics involved in producing glucose isomerase, it is of the utmost importance to use the isomerase under conditions whereby maximum yields of fructose are produced using minimum quantities of glucose isomerase. Moreover, the conditions for isomerization should be such that minimal quantities of objectionable by-products are produced.